What causes the ‘cracking’ sound during an adjustment?

The sound is no reason to be concerned. The sound is not coming from the bones themselves. It’s coming from a lubricant called synovial fluid that’s found in every joint. That fluid contains dissolved gases. During the adjustment the joints are slightly separated to move into proper position. This releases the built-up pressure on the fluid and gasses and creates a ‘pop’ and not all adjusting techniques produce the noise, which means an adjustment can be proper and effective, even if you do not hear any noise.